Authorities say 41-year-old Flavio Godoy stuck a syringe in his wife's arm while she was bent over packing her clothes.
They say he then drank the same liquid from a cup and died.
Erlendy Flores, 35, died on Tuesday.
A fire in their home next door had forced them to move into the building.
Neighbors say the bank would not give them a loan to help them out of their troubles and their situation often led to angry words.
"They were bickering and fighting and police were called about three times to the building because of domestic violence," a neighbor said.
Police say on Sunday the couple got into an argument. It was so bad, Flores ran to her sister's apartment who lives in the same building and the family called 911. Officers filed a domestic incident report and left.
The next day, while her brother waited outside the apartment, Flores returned to pick up her things. Her husband was waiting.
Police say when she bent over to pick something up, Godoy pulled out a syringe and attacked.
"The male injected into the arm of the victim with cyanide poisoning in the syringe." Police Commissioner Ray Kelly said.
Kelly said Godoy then swallowed a cup of the killer brew.
Police are investigating how Godoy got the poison. They say there was a history of domestic violence.